Surface Activated Bonding Research Articles

Ultra-Thick Metal Ohmic Contact Fabrication Using Surface Activated Bonding

: We successfully bonded aluminum foils to p+-Si substrates to fabricate Al-foil/p+-Si junctions by surface activated bonding (SAB). The electrical properties of the junctions were investigated by measuring their current voltage (I-V) characteristics. The I-V characteristics of the junctions showed excellent linearity properties. The interface resistance of the junctions was found to be 7.8 Ω·cm2. Furthermore, the resistance decreased with increasing annealing temperature and decreased to 2.4 Ω·cm2 after the junction annealing at 300 °C. These results demonstrated that thick metal ohmic contact in devices could be realized by SAB. Introduction: It is well known that parasitic resistance substantially degrades the overall performance of the electronic and optical devices in the form of contact resistance [1]. The major loss of performance is usually due to high resistance metal-semiconductor “ohmic” contacts. Therefore, in order to attain optimum device performance, the minimization of the contact resistance is absolutely necessary. The primary factors determining contact resistance are carrier concentration, semiconductor surface preparation and cleaning, and contact metal work functions hence Schottky barrier height [2]. In addition, it was reported that the ohmic contact resistance decreased with increasing the metal thickness [3]. However, the deposition of thick metal is difficult to obtain because it would take a large quantity of time and the production cost by the conventional coating method such as Electron-Beam-Evaporation and Sputter-Deposition. Junctions with different materials in lattice constants, thermal expansion coefficients, and crystal structures could be fabricated by SAB [4]. Furthermore, we previously fabricated Al-foil/p-Si Schottky diodes and investigated their electrical properties [5]. It was found that the reverse leakage currents of those diodes were decreased and their ideality factors were improved by annealing them at high temperatures. In this work, we demonstrated the potential application of metal foils as thick ohmic contacts on semiconductor substrates by applying the SAB for bonding Al foils to Si substrates. Experiments: B-doped (100) p-Si substrates and aluminum foils (Al) with a thickness of 38 μm (it is commercially available) were used for the bonding experiment. The resistivity and carrier concentrations were estimated to be 0.0034 Ω·cm and 2.64 × 1019 cm-3 respectively, by the hall measurements at room temperature. The Al/Ni/Au multilayers were evaporated on the back surfaces of p+-Si substrates prior to the bonding, and the ohmic contacts of p+-Si substrates were achieved by rapid thermal annealing at 400 °C for 1 min in N2 gas ambient. Al foils were attached to the p+-Si substrates by using SAB [4], so that Al-foil/p+-Si junctions were obtained. After the bonding, a 4-by-6 mesa array was fabricated on the Al foils by using Al wet etching for 12 h. The wide and length of the mesas were all 1.4 mm. I-V measurements were performed using an Agilent B2902A Precision Measurement Unit at room temperature. Results: Figure 1 shows the I-V characteristics of Al-foil/p+-Si junctions without and with annealing at 100, 200, 300, 400 °C measured at room temperature. We found that, in the bias voltage range for measurements, the I-V characteristics of all the junctions showed linearity. The interface resistance were found to be 7.8 Ω·cm2 for the junctions without annealing, by least-square fitting at approximately 0 V. Furthermore, the interface resistance decreases with increasing annealing temperature. It was found that the interface resistance was obtained to be 2.4 Ω·cm2 after the junction annealing at 300 °C. The interface resistance should be further decreased by optimizing the condition of SAB process and increasing the annealing temperature. These results indicate that thick electrodes with a thickness of several-tem micrometers are obtained by apply SAB. Fig. 1. I-V characteristics of Al-foil/p+-Si junctions without and with annealing at 100, 200, 300, and 400 °C measured at room temperature.

Room Temperature Bonding with Lift-Off Resist Using the Surface Activated Bonding Method for a Layer Transfer Platform

This study demonstrates the practicability of surface activated bonding for polymethylglutarimide (PMGI), as a temporary wafer bonding method at room temperature for future material integration. A bonded area covering over 92% of the wafer surface, with a room temperature bond strength of ~2 J/m2 is achieved using the bonding method. To overcome the fundamental transistor-scaling limit, novel material integration methods such as the 3D stacking are being researched. There is a growing interest in temporary bonding as one of the key technologies for 3D stacking. Recently, a wafer bonding method for polymethylglutarimide (PMGI) has been proposed for layer transfer. It has been demonstrated that the PMGI layer can be effectively bonded using thermal compression bonding. Moreover, debonding using N-Methyl-2-pyrrolidone (NMP) results in negligible amounts of the PMGI sacrificial layer as a residue on the debonded wafer surface and is therefore suitable for a transfer process.However, the high temperature annealing process in the bonding process requires a temperature of 250ºC to bond most of the loaded area; this high temperature process may damage the transferred material. Moreover, the high temperature process limits usage in applications such as integration of black phosphorous. For low temperature bonding, a surface activated bonding (SAB) process has been proposed as a room temperature bonding method. It has been proposed that polymer materials could be bonded using this process with thinly deposited layers called nano-adhesion layers at room temperature. In this study, PMGI layer is bonded to a Si substrate using the SAB method at room temperature (around 25 ºC); this method is intended to be used as a layer transfer process. One Si substrate is coated with PMGI layer, and the substrate is bonded to other Si support substrate via a proposed modified method of SAB. The proposed room temperature bonding process consists of five steps: spin-coat and soft-bake, bake in a vacuum, ion beam supper-deposition, surface activation, and bonding. Using the proposed method, a bonded area covering over 92% of the wafer surface, with a room temperature bond strength of ~2 J/m2, is achieved. It is concluded that soft-baking at 120ºC is most suitable for obtaining the strongest bond as well as the largest bonded area. Moreover, even the samples that aren’t baked after spin-coating achieved a bonded area of 81 %. This is because the solvent cyclopentanone is evaporated in the bake-in-vacuum step. The surface profile viewed using an atomic force microscope revealed that the PMGI surface was sufficiently smoothened for bonding at room temperature, after the vacuum baking process. It has been reported that it is difficult to bond a rough surface using room temperature bonding with SAB. The root means square (RMS) values of PMGI surfaces before and after vacuum annealing are 1.5 nm and 0.5 nm, respectively. The results of the AFM indicate that the surface becomes smoother after baking in a vacuum. This smooth surface is one the reasons for a considerable bonded area as well as a strong bonding strength using the proposed bonding technique. To investigate the chemical reaction between the PMGI and the deposited Si layer caused by the bonding process, attenuated total reflectance/Fourier-transform infrared spectroscopy (ATR/FT-IR) was performed on the PMGI layers before and after the deposition of the Si nano-adhesion layer. On comparing these spectra, the following features were observed: The first feature is that the peak height derived from the N-H bond that was at around 1550 cm-1, decreased after the Si deposition procedure. The second is that the peak from the C=O stretch band at around 1700 cm-1also decreased. From these results, it can be assumed that during the Si deposition process, a considerable modification of the N–H and C=O groups of the PMGI occurred after the Si deposition. This chemical modification is the key for the strong bonding of the PMGI at room temperature. In summary, we demonstrated that PMGI bonding at room temperature is capable of providing large and strong bonding interfaces. This process can be applied for 3D integration and material integration, without thermal damage. Thus, the proposed method can provide a platform for a layer transfer process suitable for 3D material integration of thin-layered systems like black phosphorous. Figure 1

Determination of Band Structure at GaAs/4H-SiC Heterojunctions

Introduction: Heterojunctions are widely used for fabricating high performance electrical and optical devices. The heterojunction fabricated by conventional epitaxial growth generally introduces a large number of crystalline defect densities at the interface due to the large differences in crystal lattice and thermal expansion coefficient. To solve these difficulties a variety of wafer bonding methods have been employed to fabricate semiconductor hetero-structure. Among them the most effective candidate is surface activated bonding (SAB) [1, 2]. In this method, surfaces of substrates are activated by the irradiation of Ar fast atom beams prior to bonding and consequently bring into contact in a vacuum condition. It enables us to fabricate heterojunctions composed of dissimilar materials with large mismatched lattice constants. We previously fabricated p-Si/n-GaAs and p-Si/n-SiC heterojunctions and investigated their electrical properties [3, 4]. It was found that the energy band diagram of p-Si/n-GaAs and p-Si/n-SiC junctions revealed type-II and type-I features, respectively. Furthermore, their conduction band discontinuities were calculated to be 0.59 and 0.3 eV, respectively. If SAB-based GaAs/SiC heterojunction are realized, the conduction band discontinuity of less than 0.3 eV should be expected, which should be suitable for fabricating devices with excellent high-frequency, high-temperature, and high-power performances that have not yet been achievable by each component material alone. In this work, we fabricated p+-GaAs/n-SiC heterojunction by using SAB method and have investigated the annealing temperature dependence of the band structure of GaAs/SiC heterojunction. The influence of the thermal annealing process on the electrical properties of the junctions were systematically investigated by measuring their current-voltage (I-V) and capacitance-voltage (C-V) characteristics. The applicability of GaAs/SiC heterojunctions for functional devices was explored based on these measurements. Experiments: p+-GaAs (100) epitaxial substrates (400 nm, ~ 1 × 1019 cm-3 epitaxial layer / substrate ~ 1 × 1018 cm-3) and n-4H-SiC epitaxial substrates (2.8 mm, 1.1 × 1017 cm-3 epitaxial layer / 0.5 mm, > 2 × 1018 cm-3 buffer layer / substrate ~ 1 × 1019 cm-3) were used for the bonding experiment. Before bonding Al/Ni/Au multilayers were evaporated on the backside of n-SiC substrates. The ohmic contacts of n-SiC substrates were formed by a rapid thermal annealing at 1000 °C for 60 s in N2 gas ambient. p+-GaAs epitaxial substrates and n-SiC epitaxial substrates were bonded to each other by using SAB [1, 2]. After the bonding, AuZn/Ti/Au multilayers were evaporated on the bottom surfaces of p-GaAs substrates. The p+-GaAs/n-SiC heterojunctions were annealed separately at 100, 200, 300, and 400 °C for 60 s in N2 gas ambient. All the samples were diced into 4 mm2 pieces. The characteristics of these samples were also investigated while the junction was unannealed. Their I-V and C-V characteristics were measured using an ADCMT 6242 Source Measurement Unit and an Agilent E4980A Precision Impedance Analyzer, respectively. Results: The 1/C2-V characteristics measured at room temperature and a frequency of 100 kHz are shown in Figure 1. The characteristics indicated a straight line and the flat-band voltages () were found to be 1.12, 1.12, 1.12, 1.13 and 1.29 V for the unannealed junction and junctions annealed at 100, 200, 300, and 400 °C, respectively, by linearly extrapolating 1/C2 to zero. It is noteworthy that the flat-band voltages remained constant as the annealing temperature increased up to 200 °C and then increased as the annealing temperature increased from 300 to 400 °C. Using the slopes of 1/C2-V characteristics, the donor concentrations of the n-SiC epitaxial layer were estimated to be 8.14 × 101 6, 8.02 × 1016, 7.98 × 1016, 7.93 × 1016, and 6.94 × 1016 cm-3, for the unannealed junction and junctions annealed at 100, 200, 300, and 400 °C, respectively, which are close to the norminal value of 1.1 × 1017 cm-3, as determined by Ni/n-SiC Schottky diode. Figure 1. C-V characteristics of p+-GaAs/n-SiC heterojunctions without being annealed and annealed at 100, 200, 300, and 400 °C measured at room temperature.

Modified Surface Activated Bonding Using Si Intermediate Layer for Bonding and Debonding of Glass Substrates

The Liquid Crystal (LC) displays using ultra-thin glass substrates of under 100 µm thick are necessary to realize the next generation display devices including the touch screen of the mobile phone, the tablet electronics and the flexible display. However, the ultra-thin glass substrates are easily deformed and damaged during the display process such as the color filter, LC and Thin Film Transistors (TFTs) at 350-550°C. Therefore the handling of the ultra-thin glass substrates in the process is a key issue for the limitation of the thickness of the display devices. In these days, in order to realize the thin display devices, the glass thinning process by chemical etchant such as HF is widely employed for the fabrication process. In this method, thick and rigid glass substrates are used during the LC display process not to deteriorate the handling quality. After assembling and sealing process, the fabricated displays are immersed in HF and thinned to the target thickness. Although this method has been successfully developed, HF is very toxic and a big environmental load. Therefore, an alternative method to handle the ultra-thin glass substrates without HF process is desirable for the display fabrication. As an alternative method to the glass thinning method, the handling by the carrier glass is has been proposed. The carrier glass is popular way to handle thin and fragile substrates in manufacture and it will enable the handling of the ultra-thin glass substrates in the process and realize the thinner display devices than by the current process without HF. In order to use the carrier glass, the ultra-thin glass should be bonded to the carrier glass and the bonding needs to be endurable for the high temperature process of TFT. Additionally, the ultra-thin glass must be released from the carrier glass after the fabrication process. However, there is no suitable method to bond the glasses and debond after high temperature process. For example, the indirect bonding method such as the adhesive glue cannot keep the bonding during high temperature process and the residue of the adhesive after debonding becomes a problem for the process and the quality of the display. The direct bonding methods like thermal compression bonding and hydrophilic bonding are successfully employed in the manufacturing process of electronics, however, the bond strength is so high that the debonding without mechanical damage to the glass substrates is not possible especially after high temperature process. In this study, we proposed applying Surface Activated Bonding (SAB) method to the bonding and debonding of the glasses for the fabrication of LC display. In the SAB method, the bonding surfaces are activated by Ar ion beam and bonded by atomic force in high vacuum at room temperature. Recently, the SAB method succeeded in bonding of glass using Si intermediate layers of under 20 nm thick which are deposited on the bonding surfaces by ion beam sputtering, nevertheless, the bond strength is too high to debond. In this case, we changed the conventional SAB method using Si layers for glass and controlled the bond strength before after heating. The Si layer was formed on one side of the glass surface in the contrary to the conventional SAB form the Si layers on the both side of the bonding surfaces. After deposition of Si, the bonding surfaces were exposed to nitrogen gas and bonded in vacuum. The bond strength was evaluated by blade insert test and was around 0.9 J/m2. The bonding endured heating at 350-550°C for 90 min and the bond strength after heating was around 0.5 J/m2. The bond strength by this method decreased after heating, whereas generally the glass bonding gets stronger after heating by the other direct bonding method. The decrease of the bond strength enables an easy debonding after the high temperature process in the display fabrication. The small voids can be seen in the bonding interface and this is the reason why the bond strength is controlled. It is indicated that the small voids are derived from water and OH groups on the bonding surfaces that were formed during the exposure to nitrogen gas, and the decomposed OH groups formed the small hydrogen bubbles after heating as a similar process to hydrophilic bonding. In this study, we succeeded in bonding the glass substrates and controlling the bond strength to achieve an easy debonding for the display fabrication. This technique will realize the thinner display devices than by current manufacturing method and reduce the environmental load.

(Invited) High Output Power Deep Ultraviolet Light-Emitting Diodes with Hemispherical Lenses Fabricated Using Room Temperature Bonding

AlGaN-based light-emitting diodes (LEDs) are attractive candidates for next-generation light sources in the deep ultraviolet (DUV) region because they can cover the spectral range of 210 – 365 nm through control of the Al content in the epitaxial layers. Moreover, they are environmentally friendly, having zero mercury content and a long lifetime. These excellent properties of AlGaN-based DUV LEDs are suitable for sterilization and polymer curing applications, however, their output power is not yet sufficient for practical applications. One method to improve output power of DUV LEDs is the enhancement of the light extraction efficiency from the LEDs chips to the air by connecting graded refractive index medium outside the chips. The refractive index medium should have the long-term reliability against DUV. We designed and fabricated the hemispherical lensmade of inorganic materials as a refractive index medium, and bonded the hemispherical lenses to the LEDs using room temperature bonding methods. We demonstrated high output power 255 nm and 280 nm LEDs with lenses [M. Ichikawa et al., APEX, to be contributed]. For this study, the excellent properties of fabricated LEDs with lenses are reviewed in connection with the bonded interface structure in light of new experimental data. The following figures (A) and (B) show schematic illustrations of LED chips and hemispherical lenses. We fabricated AlGaN-based 255 nm and 280 nm LEDs on sapphire substrates with a size of 1 mm square. A flip-chip type LED was produced to extract DUV light through the sapphire substrate. A simulation study predicted that the refractive index of the hemispherical lens was optimum when the lens was made of sapphire, which was the same material as that of LED substrates. Moreover, the light extraction efficiency was most enhanced when the lens diameter exceeded 3 mm. Therefore, we fabricated hemispherical lenses made of sapphire, with diameters of 3 or 5 mm. The LED chips were bonded to sapphire lenses at room temperature using two different bonding methods as shown in figure (C). One was surface activated bonding (SAB). In this method, the bonding surfaces of sapphire are sputter-etched and activated using an Ar fast ion beam. Transmission Electron Microscopy (TEM) images revealed that the surfaces of sapphire are consisted of several nanometer-thick amorphous layers, which were the damaged layers of the sapphire by the Ar ion beam irradiation. A large loading force was necessary to bond sapphire to sapphire. However, no light absorption was observed at the SAB interface. Another bonding method was atomic diffusion bonding (ADB). In this method, metal films are fabricated on two flat sapphire surfaces using sputter deposition with subsequent bonding of the two films on the sapphire in vacuum. Very thin Al films were used as the metal films for this study because the extinction coefficient of aluminum is small. A light absorption was slightly observed at the bonded interface in the DUV region. However, sapphire could be bonded without applying too high loading pressures. Moreover, ADB is applicable to bonding any mirror-polished substrates, in addition to sapphire, which is expected to extend the applications. The LEDs with lenses had higher light extraction efficiency than conventionally structured LEDs without lenses. In SAB devices, the maximum external quantum efficiency of the 255 nm LED was 4.56% at 100 mA, and that of the 280 nm LED was 10.1% at 1 A. As a result, at a forward current of 350 mA, the output power of a 255 nm LED with a lens increased by a factor 2.8, reaching 73.6 mW. Furthermore, that of the 280 nm LED increased by a factor of 2.3, reaching 153 mW. The output powers of ADB devices were also significantly higher than those of conventionally structured LEDs without lenses but were slightly lower than those of SAB devices because of a light absorption at the bonding interface. A further enhancement of output powers of ADB devices can be expected by tuning bonding process. The findings in this study would contribute not only to the research in DUV LEDs but also to the research of visible light LEDs. Figure 1

Bonding of SiO2 and SiO2 at Room Temperature Using Si Ultrathin Film

In three-dimensional integration technology, the bonding of metal electrode and insulator hybrid interface is very important technique. The hybrid interface serves as both an electrical connection and mechanical bond. However, the bonding of these hybrid interfaces is a challenging issue. Since the conventional bonding process requires high-temperature annealing, there exist various problems such as thermal damage, low throughput, and low alignment accuracy. As the surface activated bonding (SAB) is a bonding method carried out at room or low temperatures [1], the bonding method is expected to solve these problems. By the SAB method, direct bonding metal materials such as Cu or Al is easy. However, it is very difficult to directly bond insulator materials such as SiO2 or SiN [2], because these surfaces are rendered inactive immediately after being activated. We have reported on the bonding technique at room temperature using only Si ultrathin films for insulator materials, and we have shown that high bonding strength is achieved [3]. The surface of electrode is also covered with Si film by this bonding method. Thus, it is very important to reveal the influence of this Si film thickness on the SiO2/SiO2 bonding. In this report, we have investigated the relationship between the SiO2/SiO2bonding strength and the thickness of Si ultrathin film. Figure 1 shows a schematic illustration of the bonding apparatus (Mitsubishi Heavy Industries, Ltd., MWB-08AX) used in this experiment. Surface activation is carried out by an Ar fast atom beam (FAB). In the normal SAB process, the upper and lower wafers are irradiated with the Ar-FAB at the same time. The bonding procedure which we propose for hybrid bonding is as follows. The one of bonding wafers is held by the electric static chuck (ESC), and Si blanket wafer is placed on the lower side as the sputtering target. First, only the lower wafer is irradiated with FAB1 (first irradiation) after activating electrode surface of the upper wafer. The blanket Si wafer is then exchanged with the other bonding wafer. Only the upper wafer is irradiated with FAB2 (second irradiation) after activating electrode surface of the lower wafer, and the Si film surface on the upper wafer is also etched by FAB2. A Si thin film is then deposited on the lower wafer surface. The upper wafer and the lower wafer are optically aligned and then bonded with high load.Figure 2 shows the relationship between the surface energy and the Si ultrathin film thickness for the bonding of Si blanket wafers (8 in) with a thermal oxide. Applied voltage and current of Ar beam source operated in these experiments were about 1.8 kV and 100 mA, respectively. The background vacuum pressure is about 2×10-6Pa. The surface energy of wafers was about 1 J/m2 for thicker than about 3 nm. A cross-sectional TEM image of SiO2/SiO2bonding is shown in Fig. 3. The bonded wafers were treated with a FAB1 irradiation time of 5 min and a FAB2 irradiation time of 1 min. No micro voids were observed, and Si intermediate layer, of which the thickness is about 3nm, was seen at the bonding interface.In conclusion, we showed that high bonding strength was achieved in SiO2/SiO2 bonding with the Si film thickness of about 3 nm by this bonding method. Reference:[1] H. Takagi, K. Kikuchi, R. Maeda, T. R. Chung, and T. Suga, Appl. Phys. Lett. 68, 2222 (1996).[2] H. Takagi, R. Maeda, T. R. Chung, and T. Suga, Sensors and Actuators, A 70, 164 (1998).[3] J. Utsumi, K. Ide, and Y. Ichiyanagi, Jpn. J. Appl. Phys., 55, 026503 (2016). Figure 1

Suppressed Self-Heating in Multi-Finger InP-Based DHBTs with Au Subcollector Fabricated on SiC Substrate by Surface-Activated Bonding

InP-based double-heterojunction bipolar transistors (DHBTs) have demonstrated excellent high-frequency performance suitable for submillimeter-wave ICs and wideband-communication’s ICs. For these ICs, increasing the operation current density of the transistor is effective for improving the high-frequency performance. Multi-finger transistor design is very useful in reducing the IC chip size because it reduces the required number of transistors. However, it results in the increased power dissipation per unit area, leading to an increase in junction temperature and thus to the degradation of current gain of DHBTs. In order to suppress these undesirable effects, we propose the multi-finger DHBT on a highly thermal-conductive SiC substrate fabricated by surface-activated bonding (SAB). The measured thermal resistance, R th, shows the significant impact of the proposed DHBT structure on suppressing device self-heating thanks to the highly thermal-conductive SiC substrate and a simultaneously introduced highly thermal-conductive Au subcollector. Figure 1 shows a cross-sectional view of a fabricated multi-finger DHBT. The DHBT structure is formed on a SiC substrate (thermal conductivity κ = 490 W/m/K) through an Au subcollector (κ = 320 W/m/K). For single-finger devices, we have demonstrated the marked improvement in heat dissipation [1]. Epitaxial layer structures are designed for high-current-density operation (> 10 mA/μm2). The emitter is 20-nm-thick InP. The InGaAsSb base is 20-nm thick and highly C-doped at 7 x 1019 cm-3. The collector is 60-nm-thick InP. For the contact to the Au subcollector, 10-nm-thick, highly Si-doped InGaAs is employed. Emitter fingers and base electrodes are formed on one Au subcollector. We prepared twelve types of multi-finger DHBTs with combinations of four to six emitter fingers and emitter-finger spacing of 0.9, 1.4, and 3.4 μm. The device fabrication started with the epitaxial growth of DHBT layers on a 3-inch InP substrate by metal-organic chemical vapor deposition as shown in Fig. 2. After Au had been deposited as an adhesive and subcollector material both on the DHBT layers and a 3-inch SiC substrate, they were bonded by SAB. This bonding technique can prevent thermal degradation of the epitaxial layers because of its low bonding temperature (~150oC) [2]. After the removal of the InP substrate, DHBTs with emitter widths of 0.25 μm and 0.5 μm were fabricated using the self-aligned process [3]. Figure 3 shows a scanning electron microscope image of a fabricated four-finger DHBT with an emitter finger size of 0.5 μm x 4.0 μm. DHBTs were successfully fabricated on the SiC without any serious problems associated with the bonding process. Figure 4 plots the R th for the fabricated DHBTs as a function of the emitter-finger spacing and finger number. For comparison, the R th for the conventional DHBTs without an Au subcollector fabricated on an InP substrate is also plotted. Here, the R th was estimated by following Ref. [4] and normalized by the total emitter junction area. For the conventional DHBTs on the InP, decreasing the emitter-finger spacing or increasing the emitter-finger number markedly increases the R th due to increased heat density. On the other hand, such abrupt increases are not observed for the Au-subcollector DHBTs on the SiC. These results clearly show the combination of the Au subcollector and the SiC substrate efficiently transfers the generated heat from the inside of the device to the substrate. The proposed DHBTs exhibit 68% reduction in the R th, compared with the conventional ones when the emitter-finger spacing is reduced to 0.9 μm. Consequently, the multi-finger DHBTs on the SiC possibly operate even at much higher current density. Figure 5 shows collector I-V curves for the four-finger DHBT with a 0.25-μm-wide emitter. As expected, the DHBT on the SiC successfully suppresses the current-gain lowering in the high-V C and high-I C bias regions (~20 mW/μm2). In summary, we fabricated multi-finger InP DHBTs with an Au subcollector on a SiC using the SAB technique. The DHBTs show that the R th is almost independent of the emitter-finger spacing or emitter-finger number. In addition, the fabricated DHBTs exhibit a 68% reduction of R th compared with the conventional ones. The proposed multi-finger DHBT with the SAB technique is very useful in increasing the operation speed and decreasing the chip size of ICs.

Surface Activation and Planarization with Gas Cluster Ion Beam for Wafer Bonding

Surface activated bonding (SAB) has been widely used for various semiconductor wafers. In the SAB process, atomic Ar ions bombard wafer surface to form activated surface. In order to form activated surface on materials which are susceptible to irradiation damage, we have been investigating the gas cluster ion beam (GCIB) as an alternative to atomic ion beams. Gas clusters are aggregates of several thousands of gaseous atoms. Therefore, energy per atom of a gas cluster ion can be easily reduced to several eV. Although energy/atom of a gas cluster ion is low, thousands of low-energy atoms bombard a target surface at the same location simultaneously. As a result, the energy density on the target surface became extremely high and non-linear collisions of atoms are induced. They induces unique irradiation effects of GCIB such as high-yield sputtering, surface analysis of organic materials, thin film deposition at low temperature, and surface smoothing, which are beneficial for surface activated bonding. In this paper, preliminary irradiation effects of surface activation and planarization with glancing incidence GCIBs will be reported. From in-situ XPS analysis of O1s peak intensity on Si(100) with native oxide, Ar-GCIB irradiation at 70o removed native oxide efficiently. On the contrary, mixing of native oxide by normal incidence GCIB might occur, which caused residual oxide layer formation. In addition, the surface roughness at oblique incidence showed lowest value (Ra < 0.5 nm) without ripple formations. Cross-sectional TEM observation with oblique incidence GCIB irradiation showed that the sub-surface damage was almost negligible. These results indicated the possibility of use of GCIB as low-energy ions for wafer bonding.

(Invited) Analysis of Defect Levels at GaAs/GaAs Surface-Activated Bonding Interface for Multi-Junction Solar Cells

Room-temperature surface-activated wafer bonding between bare III-V semiconductor surfaces has become a key technology for high-efficiency multi-junction solar cells, where the reduction of interfacial electrical resistance is of crucial importance for achieving highest efficiency. In the bonding process, surface cleaning using fast atom beam (FAB) of noble gas elements is vital for successful bonding but it damages the surface, resulting in numerous crystal defects at the bonded interface and increases electrical resistance. We here developed quantitative evaluation of such defects introduced by FAB treatment. The surface of n-GaAs was treated with the FAB using Ne, Ar and Kr, and Au Schottky electrodes were formed on the surfaces. Capacitance of a Schottky diode as a function of both probe frequency and DC bias allowed us to characterize both energy depth of the defects and their density profile along the physical depth from the GaAs surface. The results indicated that atoms with the smaller diameter generate high-density defects to the deeper region from the surface. When the defect density exceeding the doping level of GaAs spreads to wider than 5 nm, significant Schottky characteristics appears in the interfacial current-voltage characteristics, as suggested by simulations. Such a tendency was semi-quantitatively in good agreement with the measured current-voltage characteristics of the n-GaAs/n-GaAs bonded interfaces treated with the FAB of Ne, Ar and Kr, suggesting that the capacitance analysis of the FAB-treated surface provides us a direction for optimizing the surface-activated bonding process using FAB.As a simple structure for clarifying the impact of SAB process conditions on the electrical conduction property of the bonded interface, two n-GaAs wafers were bonded by SAB process. Prior to the bonding process, AuGe/Ni ohmic electrode was formed on the backside of n-GaAs wafers. The wafers were then diced into chips and their surfaces were treated with the FAB of Ne, Ar and Kr in a high-vacuum chamber, with an acceleration voltage of 1.4kV and a duration of 3 min. For the admittance spectroscopy to analyze the defects introduced by the FAB treatment, the n-GaAs diced substrates were taken out of the vacuum chamber and Au Schottky electrode was evaporated on the surface. For the conductivity measurement of the boded interface, two GaAs dices after the FAB treatment were put together and pressurized to complete bonding. Subsequently, the bonded dices were further cut into 2×2 mm2 chips and current-voltage characteristics were measured by 4 terminal method.From the dependence of the junction capacitance on both AC frequency and DC bias, with an assumption of a value of carrier-capture cross section by a defect, it is possible to estimate both the energy position and the density profile along the depth direction of the defects existing in the surface vicinity of the substrate, or in the depletion region of the Schottky junction. Remarkably high defect density (around 1019 cm-3) was obtained exceeding the doping level of the substrate and the depth profile indicates that the FAB of the smaller atom introduces defects into the deeper region from the surface. The energy level of the defects were estimated to be 0.8 eV below the conduction-band edge.Taking such distribution of interfacial defects into account, the current-voltage characteristics at the junction was simulated. It was predicted that the defect region thicker than 4 nm results in nonlinear, Schottky-like conduction behavior, which we should avoid in high-efficiency cells. The measured current-voltage characteristics at the n-GaAs/n-GaAs junctions by the SAB using the FAB of Ne, Ar and Kr were compered with the prediction. The tendency of Schottky-like conduction becomes more apparent in the sequence of Ne, Ar and Kr. This is the sequence of smaller atomic radius. In the defect density profile, if we focus on a certain defect density level such as 0.5×1019 cm-3, the penetration depth of the defects is larger also in the sequence of Ne, Ar and Kr.It is strongly suggested that the high-density defects at the interface by wafer bonding distorts band edges and makes potential barrier for electrical conduction. In SAB process, FAB treatment introduces the defects in 1019 cm-3 level and it has significant impact on the interfacial electrical conduction. A suggestion for reducing such introduction of defects is the use of larger atoms in FAB treatment, which reduces penetration depth of defects. It is clear that the condition of FAB such as acceleration voltage of primary ions and exposure time impacts the density and distribution of defects. The optimization of such process parameters is quite possible using the admittance spectroscopy proposed in this work. Figure 1

Combined Surface Activated Bonding Technique for Hydrophilic SiO2-SiO2 and Cu-Cu Bonding

As an evolution of the Cu-Cu bonding and SiO2-SiO2 bonding, Cu/SiO2 hybrid bonding is a promising approach to the emerging three-dimensional (3D) integration of microelectronic/photonic systems, since it obtains both direct metal interconnection and enhanced thermal/mechanical stability with a seamless bonding structure during the single bonding process.1,2 However, because of the different features of Cu-Cu and SiO2-SiO2 bonding, Cu/SiO2 hybrid bonding at low temperatures of no more than 200 °C remains challenging. For instance, the Cu-Cu thermo-compression bonding is typically conducted in vacuum or dry protecting/reducing atmospheres after removal of surface oxides,3–7 while the SiO2-SiO2 bonding needs a humid environment to facilitate termination of Si-OH bonding sites.8–12 It is highly desired to develop a new bonding process that is effective for both Cu-Cu and SiO2-SiO2 bonding in H2O-free ambient, such as vacuum, for improvement of the Cu/SiO2 hybrid bonding. Recently, we proposed a combined surface-activated bonding (SAB) method, which involves a combination of surface bombardment using a Si-containing Ar beam and prebonding attach-detach procedure prior to bonding in vacuum. High SiO2-SiO2 bonding strength of close to the Si bulk fracture strength has been realized at 200 °C. In this paper, we report our recent results of Cu-Cu and SiO2-SiO2 bonding by using the combined SAB method. The mechanism is discussed to understand the present low-temperature bonding technique.

(Invited) Surface Activated Wafer Bonding; Principle and Current Status

The surface activated bonding (SAB) is a method which achieves strong bonding at room temperature. It is assumed that at the bonding interface prepared by SAB atoms bind to each other by the same interatomic bonds with those in bulk materials, such as covalent bond, metallic bond, etc. SAB well matches the wafer direct bonding, because intimate contact at the bonding interface is spontaneously achieved by the attractive force between very smooth wafer surfaces. In SAB, the surfaces of bonded materials are sputter-etched, in other word “cleaned”, before bonding. Various materials, such as semiconductor wafers, single crystal oxide wafers, metal films on wafers, etc. has been successfully bonded. Recently, other surface treatments have been proposed to apply SAB to wide variety of materials. SAB has been used to fabricate integrated substrates for RF-filters for more than 10 years, and has been already applied in MEMS packaging in industry. Other applications, such as power semiconductors, solar cells, etc. are now extensively developed.

Room Temperature Bonding with Lift-Off Resist Using the Surface Activated Bonding Method for a Layer Transfer Platform

A room temperature bonding of polymethylglutarimide (PMGI) is performed as a temporary bonding process. The previous study showed that temporary bonding process with PMGI achieves extremely small residues on a debonded surface, but bonding temperature reaches to 250 oC. In this study, the surface activated bonding (SAB) method is applied for bonding the PMGI to the Si wafer at room temperature for heat-sensitive materials. Using SAB, a bonded area covering over 92% of the wafer surface, with a room temperature bond strength of ~2 J/m2, is achieved. For debonding, PMGI dissolve into N-methylpyrrolidone based solvent. Surface profile revealed that extremely small amount PMGI residues remains on the debonded surface. This process can be applied for 3D integration of 2D materials.

Ultra-Thick Metal Ohmic Contact Fabrication Using Surface Activated Bonding

We successfully bonded aluminum foils to Si substrates to fabricate p-Si/Al, n-Si/Al, p+-Si/Al, and n+-Si/Al junctions by surface activated bonding (SAB). The effects of the annealing temperature process on the electrical properties of the junctions were investigated by measuring their current voltage (I-V) characteristics. It was found that the leakage current of the reverse bias of n-Si/Al junctions was improved and the I-V characteristics of p-Si/Al revealed excellent linearity properties after the junctions annealing at 400°C. The interface resistance of p+-Si/Al, and n+- Si/Al junctions decreased with increasing annealing temperature and decreased to 0.021 and 0.032 Ω·cm2 after the junction annealing at 300 and 400°C, respectively. These results demonstrated that thick metal Ohmic contact in devices could be realized by SAB.

Suppressed Self-Heating in Multi-Finger InP-Based DHBTs with Au Subcollector Fabricated on SiC Substrate by Surface-Activated Bonding

InP-based multi-finger double-heterojunction bipolar transistors (DHBTs) with an Au subcollector were fabricated on a highly-thermal-conductive SiC substrate by surface-activated bonding (SAB). The fabricated DHBTs have good electrical properties without any degradation due to the wafer-bonding process. They also show 68% reduction of thermal resistance (R th) compared with a conventional DHBT on an InP substrate. Reduced R th enables us to increase current density while maintaining the junction temperature of DHBTs, which boosts their operating speed and enhances their integration density. Therefore, multi-finger DHBTs on a SiC substrate fabricated by SAB is very useful for reducing IC chip size and increasing the operation speed.

Direct Wafer Bonding of SiC-SiC at Room Temperature by SAB Method

In this work, direct wafer bonding of SiC-SiC was accomplished by surface activated bonding (SAB) method at room temperatrue. The bonding energy of ~1.4 J/m2 was obtained without orientation dependence, which is much weaker than bulk SiC strength. Correspondingly, the tensile strength of bonding interface is ~12.2 MPa. The bonding mechanisms were investigated through Monte Carlo simulation and interface analysis by transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDX). The formation of amorphous SiC during surface activation may eliminate the affects of orientation on bonding. In addition, the possible Si preferentially sputtering during surface activation is assumed to be the reason why direct wafer bonding of SiC-SiC cannot reach bulk SiC strength. After a rapid thermal annealing at 1273 K for 180 s in Ar, the tensile strength of bonding interface could be improved to the values higher than 21.6 MPa.

Cu-Cu Die to Die Surface Activated Bonding in Atmospheric Environment Using Ar and Ar/N2 Plasma

Surface activated bonding (SAB) method is an attractive option for Cu-Cu bonding as the force application duration is considerably shorter as compared to thermocompression bonding. Die to die SAB allows 3D heterogeneous integration to be done with lesser energy consumption compared to thermocompression. In this report, blanket and patterned Cu-Cu die to die SAB are demonstrated. Two different types of plasma were used for the surface activation of Cu. Both Ar and Ar/N2 plasma activation were performed on the Cu surfaces enabling direct bonding at room temperature, ambient air, and atmospheric pressure without any wet chemical process.

Bonding of SiO2 and SiO2 at Room Temperature Using Si Ultrathin Film

The bonding of metal electrode and insulator hybrid interfaces is very important in three-dimensional integration technology. Surface activated bonding (SAB) is expected to be suitable method for three-dimensional integration technology, as the bonding method is carried out at room or low temperatures. Though metal materials such as Cu or Al is easy to directly bond by the SAB method, insulator materials such as SiO2 or SiN are difficult to directly bond. We have already reported on the bonding technique at room temperature using only Si ultrathin films for insulator materials, and we have shown that high bonding strength is achieved. In this report, we investigated the relationship between the SiO2/SiO2 bonding strength and the thickness of Si ultrathin film. We confirmed that the surface energy was about 1 J/m2 for the Si film’s thickness of more than about 3nm.

Combined Surface Activated Bonding Technique for Hydrophilic SiO2-SiO2 and Cu-Cu Bonding

Wafer bonding with high quality (such as high strength, void-less interface, and low-resistance vertical electrical interconnects) obtained at low temperatures is highly desired for 3D integration of microsystems. This paper reports a combined surface activated bonding (SAB) technique for improvement of the bonding quality of SiO2-SiO2 pairs bonded at 200°C. The present bonding technique employs a combination of surface irradiation using a Si-containing Ar beam and prebonding attach-detach procedure prior to bonding in vacuum, followed by postbonding annealing in ambient pressure. The bonding method has also been found effective for SiO2-SiNx and Cu-Cu bonding at 200°C. Results of bonding strength measurements, transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDS) observations are reported and discussed to understand the present technique. The feasibility of this bonding technique for Cu/SiO2 and Cu/SiO2/SiNx hybrid bonding is also discussed.

Determination of Band Structure at GaAs/4H-SiC Heterojunctions

The effects of thermal annealing process on the interface in p+-GaAs/n-SiC heterojunctions fabricated by using surface-activated bonding (SAB) were investigated. It was found by measuring their current-voltage (I-V) characteristics that the reverse-bias current and the ideality factor were extracted to be 7.57 ´ 10-7 A/cm2 and 1.33, respectively, for the junctions annealed at 400 °C. The flat-band voltage obtained from capacitance-voltage (C-V) measurements was found to be 1.29 eV, which is almost consistent with the turn-on voltage extracted from I-V characteristics. These results suggest that the SAB-based GaAs/SiC heterojunctions are applicable for fabricating high-frequency power devices.

(Invited) Analysis of Defect Levels at GaAs/GaAs Surface-Activated Bonding Interface for Multi-Junction Solar Cells

Quantitative evaluation of crystal defects introduced by fast atom beam (FAB) treatment was developed for surface-activated wafer bonding. The surface of n-GaAs was treated with the FAB using Ne, Ar and Kr, and Au Schottky electrodes were formed on the surfaces. Capacitance of a Schottky diode as a function of both probe frequency and DC bias allowed us to characterize both energy depth of the defects and their density profile along the physical depth from the GaAs surface. The results indicated that atoms with the smaller diameter generate high-density defects to the deeper region from the surface. When the defect density exceeding the doping level of GaAs spreads to wider than 5 nm, significant Schottky characteristics appeared in the interfacial current-voltage characteristics, as suggested by simulations. Such a tendency was semi-quantitatively in good agreement with the measured current-voltage characteristics.